In a cryogenic air separation apparatus, before a distillation at a cryogenic temperature, feed air is purified to remove impurities which may block a pipe or a heat exchanger. Examples of a substance, which may block, include water, carbon dioxide, and a nitrogen oxide (for example dinitrogen monoxide). Also, it is reported that a hydrocarbon is concentrated in liquid oxygen so that it may become a cause of explosion. Therefore, it is necessary to remove these impurities from the viewpoint of safety operation and ensuring safety of a cryogenic air separation apparatus.
The removal of these impurities is performed by using a purification apparatus with an adsorption column packed with an adsorbent and using a temperature swing adsorption method (TSA method) or a pressure swing adsorption method (PSA method). In general, a PSA method has a larger energy loss than a TSA method because the switching frequency of adsorption columns is often. Also, the yield of purified gas is not good because a large amount of regenerating gas is required in a PSA method. Therefore, a TSA method is mainly used for the purification of feed air for cryogenic air separation.
In recent years, in response to the demand for a large amount of production, a cryogenic air separation apparatus is likely to be upsized. However, further improved performance is also desired in order to hold down initial investment. For example, in a purification apparatus for feed air, an adsorbent with an increased adsorption capacity has been developed in order to reduce a used amount of the adsorbent (see patent reference 1).
When the adsorption capacity is increased, the amount of an adsorbent, which is necessary to process the same amount of gas, can be decreased. Therefore, an adsorption column can be downsized. In the purification apparatus using a TSA method, an adsorbent with a small particle diameter has been used from the viewpoint of effectively using the adsorption capacity of an adsorbent. Therefore, in order to prevent an adsorbent from being blown up and fluidized, an air velocity at the entrance of an adsorbent layer is adjusted within a certain range.
There is the relationship of “(air velocity)=(flow rate of feed air)/(cross-sectional area of adsorption column)” among an air velocity at the entrance of an adsorbent layer, a flow rate of feed air, and a cross-sectional area of an adsorption column. Therefore, a cross-sectional area of an adsorption column at a certain flow rate of feed air is determined by an air velocity regardless of other conditions. Because an air velocity is constant, a column diameter is kept constant, and the downsizing of a purification apparatus due to the decrease in an amount of an adsorbent is achieved by lowering the height of an adsorption column.
Meanwhile, when a flow rate of feed air is increased in order to increase production, an amount of an adsorbent, which is necessary for pretreatment, is increased in proportion. From the aforementioned relationship between an air velocity and a cross-sectional area of an adsorption column, the increase in a flow rate of feed air corresponds to the increase in a cross-sectional area of an adsorption column, i.e. the increase in a column diameter. As previously stated, because the effect due to the improved performance of an adsorbent is lowering the height of an adsorption column, the increase of a throughput rate makes a column diameter to be larger than the height in an adsorption column.
However, two problems occur in the adsorption column with a large column diameter. One is the problem of gas dispersion, and the other is the problem of an installation area. In general, during adsorption operation, it is necessary to consider the dispersion so as to uniformly flow gas in an adsorbent layer. However, when the diameter of an adsorbent layer is larger than the packed height, it is difficult to disperse gas uniformly. Meanwhile, when a column diameter of an adsorption column is large, an installation area is large, too. Therefore, the demand of the downsizing of an apparatus is not fulfilled.
Accordingly, a conventional design method of an adsorption column has limitations as a countermeasure for upsizing of a cryogenic air separation apparatus.
Meanwhile, examples of a technique to remove a nitrogen oxide and a hydrocarbon in feed air include a method to remove them by using an adsorbent such as zeolite.
Patent reference 2 discloses a method and an equipment to remove moisture, carbon dioxide, and dinitrogen monoxide, which use the first, second, and third adsorbent layers made of three kinds of adsorbents respectively corresponding to moisture, carbon dioxide, and dinitrogen monoxide in air purification using a TSA method.
The FIG. 2 of this patent reference 2 shows that N2O breaks through much earlier than carbon dioxide in the case where air including carbon dioxide and N2O flows in a NaX zeolite adsorbent layer. The removal rate of N2O at the time point when carbon dioxide breaks through, which is calculated from the aforementioned figure, is about 30%, indicating that it is difficult to simultaneously remove carbon dioxide and N2O by using only a NaX zeolite layer in a TSA method.
NaX zeolite has the properties of adsorbing N2O and much strongly adsorbing carbon dioxide. Because N2O, which is adsorbed once, is desorbed by carbon dioxide, and pushed downstream, N2O breaks through much earlier than carbon dioxide. Therefore, patent reference 2 teaches that the third adsorbent is packed in order to remove the pushed N2O.
Patent reference 3 discloses that an adsorbent layer selected from among alumina and X type, Y type, and A type zeolites is used in a process to remove moisture, carbon dioxide, and a nitrogen oxide by a PSA method. However, by the reference to the matters described in patent reference 2, the removal of a nitrogen oxide by X type zeolite seems to be the effect limited in a PSA method, for example.
In the case of a TSA method, as the adsorption equilibrium of carbon dioxide reaches from the upstream of an adsorbent layer to the downstream, N2O is pushed downstream. On the other hand, in a PSA method, the adsorption distribution of carbon dioxide is broadened over an adsorbent-packed layer, and so it is speculated that the effect of pushing N2O does not appear as remarkably as a TSA method.
In general, a PSA method has a larger energy loss than a TSA method because the switching frequency of adsorption columns is often. Also, the yield is not good because a large amount of regenerating gas is required in a PSA method. Accompanying the upsizing of a cryogenic air separation apparatus, these demerits become significant, and the big difference occurs between two methods. Therefore, if N2O can be removed with the adsorption of carbon dioxide by a TSA method in the same way as a PSA method, the demerits of PSA method can be avoided.
[Patent Reference 1] Japanese Unexamined Patent Application, First Publication No. 2001-347123
[Patent Reference 2] Japanese Unexamined Patent Application, First Publication No. 2000-107546
[Patent Reference 3] European Patent Application, Publication No. 862,938